I've got one pigmy angel (Purple yellow type) with my baby clam, hasn't bothered it in 3 months since I had the clam. AND.. it is in a 10g nano reef. It does like to graze on everything on the LR, but my softies, LPS, SPS and clam has never had any damage or injury from THIS fish. I feed the Pigmy angel once, every other day.
Clam and all corals are doing very well, the fish is just a busy body, always dodging and hiding and finding places in the rock to investigate. He's always moving, and riding the current from the powerhead. (Shoots the rapids kids call it.)
That is just my experience. I've had the fish for a year. Any other type of angel, I wouldn't keep with a clam I think. My pigmy is 2 years old, about 2 inches long. (got it from a friend who didn't want it anymore, because she was getting a coral beauty, and didn't want any issues with the two fish)
Here is a representative pic of my angelfish..
